JP2008076297A - Evaluation method for stress corrosion cracking resistance of aluminum alloy material, and aluminum alloy material excellent in stress corrosion cracking resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an improved evaluation method, as to competence to a stress corrosion cracking resistance behavior of an actual aluminum alloy material, and the aluminum alloy material excellent in stress corrosion cracking resistance evaluated by the evaluation method. <P>SOLUTION: An aluminum alloy material test piece of an evaluation object is subjected as a C-ring test piece loaded with a prescribed rate of stress, an anode polarization curve is measured in an aqueous solution containing 5.8 mass% of NaCl adjusted to pH 10 at 30°C, by a three-electrode method, the stress corrosion cracking resistance is evaluated in a range from 1 A/cm<SP>2</SP>of current density to 10 A/cm<SP>2</SP>thereof, based on an average gradient of current/potential, and a 6000 type aluminum alloy forged material is judged to be excellent in the stress corrosion cracking resistance, when the average gradient of current/potential is 350 Ω<SP>-1</SP>M<SP>-2</SP>or less. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an evaluation method for stress corrosion cracking resistance of an aluminum alloy material and an aluminum alloy material excellent in stress corrosion cracking resistance evaluated by this evaluation method. And the evaluation method of stress corrosion cracking resistance that can be tested in a shorter time than the actual stress corrosion cracking resistance test, and corresponded well with the actual stress corrosion cracking resistance test result, and was evaluated by this evaluation method The present invention relates to an aluminum alloy material excellent in stress corrosion cracking resistance. Hereinafter, aluminum is also referred to as Al.

  Stress corrosion cracking is a phenomenon in which the corrosion of the environment and the mechanical action of the stress applied are superimposed, leading to the destruction of the metal material. It is known that stress corrosion cracking occurs when the metal material, environment, and stress satisfy certain conditions, and are generated in various combinations.

  Stress corrosion cracking occurs even when the working stress is less than the strength of the metal material (= the limit of fracture normally evaluated in a laboratory, not in the actual environment). Therefore, when determining the design strength of a structural material, it is necessary to accurately grasp the critical stress of stress corrosion cracking, and its characteristic evaluation technique is extremely important in practice. Also, in the case of material development that takes measures to prevent stress corrosion cracking of metal materials, a short-term laboratory characterization technique for stress corrosion cracking is indispensable.

  As a stress corrosion cracking test method, a constant load method, a constant strain method, and the like are known, and standardized by JIS, ASTM (American Society for Testing Material), and the like. For example, a stress corrosion cracking test method for an aluminum alloy is standardized in JIS H8711.

  On the other hand, as is well known, Al alloys such as JIS 6000 series (Al-Mg-Si series) are used for structural materials or parts of transportation equipment such as vehicles, ships, aircrafts, motorcycles or automobiles. This JIS 6000 series Al alloy is relatively high in strength and excellent in corrosion resistance, and is also excellent in terms of recyclability in which scrap can be reused as a JIS 6000 series Al alloy melting raw material.

  However, when increasing the strength of these aluminum alloy forgings, usually an excessive amount of Si is added or a strengthening element such as Cu is added. However, when the amount of excess Si is increased or a strengthening element such as Cu is included in this way, the structure of the Al alloy forging material becomes extremely sensitive to intergranular corrosion and stress corrosion cracking. Another problem arises that the corrosion resistance is reduced. And the problem of this corrosion-resistance fall becomes serious as a problem regarding durability and reliability which are the fundamental required characteristics as a structural material.

  In the structural material such as the transport aircraft, the structure is often used not only in the Al alloy forging material but also in combination with or joined to other metal materials such as iron, which is noble than the Al alloy. In addition, the environment of use includes seawater and salt water, and is in a severe salt water corrosive environment from a low temperature below freezing to a high temperature in midsummer.

  Therefore, even in such an environment of use, and as described above, even when the strength of the alloy itself is increased, the required characteristics and technical problems of excellent stress corrosion resistance are present in Al alloy forgings. is there.

In response to this problem, as a 6000 series aluminum alloy forging material having a specific composition, the size of crystal precipitates existing on the grain boundary of the structure is controlled, and further, this aluminum alloy forging material is used as an anode, and 5% at 30 ° C. A technique has been proposed in which the minimum value of the natural potential of an aluminum alloy forged material measured after direct current electrolysis at 100 μA / cm ² for 30 minutes in a NaCl solution at a concentration of% is greater than a specific value (see Patent Document 1). .

In addition, as a method for evaluating the stress corrosion resistance of 6000 series aluminum alloy forgings, this aluminum alloy forging was used as an anode and DC electrolysis was carried out at 10 to 1000 μA / cm ² for 10 to 60 minutes in 1 to 10% NaCl aqueous solution. Evaluation based on the value of the natural potential of the aluminum alloy forging material measured later is also proposed (see Patent Document 2).
JP 2002-294382 A (full text) JP 2001-99812 A (full text)

  The simple evaluation method of stress corrosion cracking resistance using the natural potential of the Al alloy forging material in Patent Documents 1 and 2 described above is as follows. First, in a NaCl aqueous solution that simulates a salt water corrosion environment such as seawater of a structural material of a transport aircraft, Forcible etching by direct current electrolysis of the Al alloy forging material is performed, and then the change over time of the natural potential of the Al alloy forging material in the NaCl aqueous solution is measured to measure the minimum value of the natural potential.

  These simple evaluation methods for stress corrosion cracking using natural potentials can be tested in a much shorter period of time than actual stress corrosion cracking resistance tests, and the measured natural potential tends to be the same as the actual stress corrosion cracking tendency. It corresponds well.

  These simple evaluation methods for stress corrosion cracking using natural potentials, however, have a point to be further improved with respect to the correspondence with the stress corrosion cracking behavior of actual 6000 series aluminum alloy forgings.

  The present invention has been made in view of the above circumstances, and the object thereof is to evaluate the improved evaluation method and the evaluation method with respect to the correspondence with the stress corrosion cracking resistance behavior of an actual aluminum alloy material. The present invention provides an aluminum alloy material and an aluminum alloy forged material that are excellent in resistance to stress corrosion cracking.

In order to achieve the above object, the gist of the present invention is an evaluation method of stress corrosion cracking resistance of an aluminum alloy material, in a state where a predetermined stress is applied to an aluminum alloy material test piece to be evaluated. The anodic polarization curve in a 5.8 mass% NaCl aqueous solution adjusted to a temperature of 30 ° C. and a pH of 10 was measured by the three-electrode method, and the current density of the measured anodic polarization curve was from 1 A / cm ² to 10 A / cm ^2. The stress corrosion cracking resistance of this aluminum alloy material is evaluated by the average current / potential gradient in the range up to.

In order to achieve the above object, the gist of the aluminum alloy material of the present invention is an Al-Mg-Si aluminum alloy material whose stress corrosion cracking resistance has been evaluated by the above evaluation method, and this aluminum alloy material In a state in which 80% of the 0.2% proof stress of the aluminum alloy material was loaded on this specimen, it was measured by a three-electrode method in a 5.8 mass% NaCl aqueous solution adjusted to a temperature of 30 ° C. and a pH of 10. In the anodic polarization curve, the average current / potential gradient in the current density range of 1 A / cm ² to 10 A / cm ² is 350 Ω ⁻¹ · m ⁻² or less.

  As the Al-Mg-Si-based aluminum alloy material, stress corrosion cracking resistance is an important issue, in mass%, Mg: 0.30-5.0%, Si: 0.20-2.0%, Cu: 0.01-2.0% , Mn: 0.01 to 1.0%, Fe: 0.01 to 1.0%, Cr: 0.01 to 2.0%, Zn: 0.005 to 10.0%, respectively, and a forging material composed of the balance Al and inevitable impurities is preferable.

  And, as this Al-Mg-Si based aluminum alloy forging material, further, in mass%, Ti: 0.001-0.5%, B: 0.0001-0.05%, Nb: 0.01-1.0%, Zr: 0.01-1.0% V: One or two or more selected from 0.01 to 1.0% may be contained.

  The present invention presupposes the stress corrosion cracking evaluation method using natural potential measurement in a NaCl aqueous solution simulating the salt water corrosion environment of the aluminum alloy forging material in Patent Documents 1 and 2 described above.

  However, in the present invention, the natural potential is measured by the three-electrode method using an aluminum alloy material specimen subjected to a load stress as an anode, and the slope of the anode polarization curve in the specific current density range, that is, the current / It is characteristic that stress corrosion cracking resistance is evaluated by the average gradient of potential.

  As a result, the present invention, like the method for evaluating stress corrosion cracking in Patent Documents 1 and 2 above, evaluates stress corrosion cracking resistance in a short period of time, and designs a structure for improving stress corrosion cracking resistance. It is possible to speed up material development.

  In addition, with regard to the correspondence with the stress corrosion cracking resistance behavior of actual aluminum alloy materials, there is an improved evaluation method that has more correspondence and correlation, and aluminum alloy forging with excellent stress corrosion cracking resistance evaluated by this evaluation method. Can provide material.

  Hereinafter, a stress corrosion cracking resistance evaluation method according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a conceptual diagram of a measuring apparatus used in the evaluation method of the present invention.

(measuring device)
FIG. 1 shows an aspect in which an anode polarization curve is measured by applying a predetermined ratio of stress to a C-ring type test piece made of an aluminum alloy material to be evaluated to form an anode electrode.

  In FIG. 1, 1 is a container, 2 is NaCl aqueous solution with which the container was filled. Reference numeral 3 denotes a C-ring type anode electrode test piece of an aluminum alloy material to be evaluated under stress, and reference numeral 4 denotes a cathode electrode as a counter electrode. 5 is a reference electrode, and 6 is a polarization curve measuring device. These electrodes 3, 4, 5 are all immersed in the NaCl aqueous solution 2.

As the container 1, a material such as a glass container or a plastic container that is not easily affected by the NaCl aqueous solution is selected. In addition, the temperature of the NaCl aqueous solution is adjusted using a water bath, a throw-in heater, or the like. For the cathode electrode 4, a counter electrode used for usual electrochemical measurement, such as a platinum plate or carbon, can be used. The area of the counter electrode 4 is 1 cm ² or more. As the reference electrode 5, an electrode used for usual electrochemical measurement such as a silver / silver chloride electrode or a saturated calomel electrode can be used. Further, the reference electrode 5 may be used by being immersed in a container different from the container 2 filled with the NaCl aqueous solution through a salt bridge in order to make it difficult to be attacked by the NaCl aqueous solution.

  As the polarization curve measuring device 6, a normal polarization curve measuring device such as a potentiostat or a function generator can be used alone or in combination. The obtained potential and current value (current density) can be collected by a personal computer or a data logger.

(Current / potential gradient measurement)
In the apparatus configuration as described above, by increasing the voltage (potential) with respect to the reference electrode 5 from 0 V with time, the test piece 3 of the aluminum alloy material to be evaluated is subjected to an electrolysis action to generate a current. This electrolytic action is the same behavior as stress corrosion cracking in the actual use environment of the aluminum alloy material.

  When the test piece 3 is highly susceptible to intergranular corrosion and stress corrosion cracking, hydrogen ions are generated at the tip of the test piece 3 due to corrosion and dissolution of Al ions, and Cl ions As the concentration becomes remarkable, a phenomenon occurs in which the current density representing the dissolution rate of the test piece 3 increases remarkably with respect to the potential.

In the present invention, this current increase phenomenon is detected by the dynamic potential method, and the current density of the C-ring type anode electrode test piece (evaluation target aluminum alloy material test piece) 3 loaded with stress as sensitivity to the stress corrosion cracking property is as follows. in the range of ^{1mA / cm 2 ~10mA / cm 2} , the slope of the current / potential - taken in a (potential gradient of the current curve).

(Potential feed rate)
At this time, the rate of increasing the potential (potential feed rate) is preferably in the range of 20 A / cm ^{2 to} 100 mA / cm ² . The potential feed rate is more preferably in the range of 30 A / cm ^{2 to} 80 mA / cm ² . In the current / potential gradient measurement, if the potential feed rate is too large, the dissolution reaction of the test piece cannot catch up with the potential change. For this reason, an accurate potential-current relationship cannot be obtained. On the other hand, if the rate of increasing the potential (potential feed rate) is too small, the time required for the measurement becomes longer, and the dissolution reaction is relaxed by the accumulation of corrosion products. The relationship cannot be obtained.

Fig. 2 shows the anodic polarization curve (potential-current) when DC electrolysis was performed in a 5.8 mass% NaCl aqueous solution adjusted to a temperature of 30 ° C and pH 10 by the three-electrode method using these aluminum alloy specimens as anodes. (Curve) is an example measured by the potentiodynamic method. In this example, the potential feed rate is 50 A / cm ² .

In FIG. 2, the aluminum alloy material A has a relatively gentle average current / potential gradient in the range of current density from 1 A / cm ² to 10 A / cm ^2, and is 350 Ω ⁻¹ · m ⁻² or less. On the other hand, in the case of aluminum alloy material B, the average current / potential gradient in the current density range from 1 A / cm ² to 10 A / cm ² exceeds 350 Ω ^-1 · m ^-2 . . Therefore, the aluminum alloy material A has a low sensitivity to stress corrosion cracking, and the aluminum alloy material B has a high sensitivity to stress corrosion cracking.

  That is, it can be said that the greater the current / potential gradient (also referred to as the slope of the anodic polarization curve or the potential-current curve), the higher the sensitivity to stress corrosion cracking and the lower the stress corrosion cracking resistance. On the other hand, it can be said that the smaller the current / potential slope (the slope of the potential-current curve), the lower the sensitivity to stress corrosion cracking and the better the stress corrosion cracking resistance.

In other words, if the current / potential average slope is relatively gentle and the current density is less than 350 Ω ^-1 · m ^-2 in the current density range of 1 A / cm ² to 10 A / cm ^2, Acceleration of intergranular corrosion and intergranular cracking due to hydrogen ion generation and Cl ion concentration hardly occurs, and the stress corrosion cracking resistance 3 of the evaluation test piece is excellent.

  As described above, in the present invention, the current / potential of the test piece 3 is measured because the measurement condition captures (reflects) the tendency of current increase behavior of an aluminum alloy material due to stress corrosion cracking. The slope of anodic polarization (the slope of the anodic polarization curve, the slope of the potential-current curve) corresponds well to the stress corrosion cracking resistance of the actual aluminum alloy material.

It should be noted that the average gradient of current / potential in this current density range from 1 A / cm ² to 10 A / cm ^2, which is the standard value for stress corrosion cracking resistance evaluation, is the component composition (alloy element) of the aluminum alloy. Type and content) and the structure (size, number, crystal grain size, etc. of crystallized and precipitated grains at grain boundaries and grains). As a result, the type of alloy of the aluminum alloy material to be evaluated, the type of wrought material, or the type of the same alloy or type of wrought material are greatly different depending on the manufacturing conditions.

On the other hand, in the case of the same Al-Mg-Si (6000 series) aluminum alloy material, the current / potential in this current density range from 1 A / cm ² to 10 A / cm ² including forgings. If the average slope of the material is relatively gentle, if it is 350 Ω ^-1 · m ^-2 or less, the acceleration of intergranular corrosion and intergranular cracking occurs due to hydrogen ion generation and Cl ion concentration at the corrosion front in the actual environment. The evaluation test piece 3 has excellent stress corrosion cracking resistance. On the other hand, if the current / potential average gradient exceeds 350 Ω ^-1 · m ^-2 in the current density range of 1 A / cm ² to 10 A / cm ² , it is susceptible to stress corrosion cracking. Is high and inferior in stress corrosion cracking resistance.

Hereinafter, a specific method for measuring the anodic polarization curve will be described.
(Stress load test piece)
As shown in Fig. 3, the test piece 3 to which the load stress is applied is a C-ring test piece with an outer diameter of 19 mm described in Annex 5 of JIS H8711. The load stress is fixed by tightening the bolt 6. It shall be loaded under conditions. The load stress means a stress value at the maximum stress point (vertex) of the test piece 3, and it is preferable to attach a strain gauge to the apex portion of the outer surface and control the load stress by tightening the bolt 6.

  An area having a width of 5 mm is defined as an electrode surface with a line passing through the apex of the outer surface of the C-ring test piece 3 as a center line, and the other surface is covered with a silicon sealant or the like. This is for measuring the gradient of the anodic polarization curve only in the region where the stress is actually applied in the C-ring test piece 3 subjected to the load stress.

As a pretreatment of the test piece 3 to which the applied stress is applied or the test piece 3 to which the applied stress is not applied, for reproducibility, it is commonly immersed for 15 seconds in 10% NaOH (70 ° C) and washed with water. as the condition of immersion for 30 seconds in 30 wt% HNO ₃ (30 ℃) after performing.

(Load stress)
This load stress corresponds to and correlates with the stress corrosion cracking resistance of the actual aluminum alloy material, and in order to make the measurement reproducible, during the measurement, it is the same as the 0.2% proof stress value of the aluminum alloy material. The load stress is a percentage (same, constant percentage). In order to better correlate the load stress, it is preferable to determine it equivalent to the stress applied in the environment (use) of the aluminum alloy material.

  In this regard, in the case of Al-Mg-Si (6000 series) aluminum alloy forgings, this is matched to the suspension arm as an undercarriage member for automobiles, etc., which has the highest stress in the usage environment (application). The applied stress is preferably 80% of the 0.2% proof stress value of the aluminum alloy material.

  As for the stress loading method on the test piece, in addition to the C-ring test piece, a U-bend test piece, a 3-point bend test piece, a 4-point bend test piece, etc., which are generally used in the past, are bolted. Although there is a stress applying method for applying a stress of 2 mm, it is specified as a C-ring test piece in the present invention in consideration of reproducibility.

(Test solution)
Test solution 2 used for the measurement of the anodic polarization curve is a 5.8 mass% NaCl aqueous solution adjusted to pH 10 (± 0.2) with NaOH. Adjust the NaCl aqueous solution temperature to 30 ° C with a water bath or the like, and perform measurement under open air conditions. The specific volume should be at least 300 mL per specimen.

  Stress corrosion cracking often occurs in an environment containing the above-described stress load, particularly chloride ions. Therefore, in order to simulate the corrosive environment in which the aluminum alloy material is used, the test solution 2 is a NaCl aqueous solution having the above specific concentration for reproducibility.

At this time, if the pH of the test solution 2 becomes too low, that is, if the H ⁺ ion concentration becomes high, the difference in H ⁺ ion generation due to the difference in stress corrosion cracking sensitivity does not appear as a difference in current gradient. For this reason, the case where the stress corrosion cracking sensitivity (stress corrosion cracking characteristic) of a test piece (aluminum alloy material) cannot be accurately evaluated may occur. On the other hand, if the pH is too high, on the other hand, overall corrosion may be accelerated or alkali brittle cracks may occur, and the stress corrosion cracking characteristics of the test piece (aluminum alloy material) may not be accurately evaluated. Therefore, in the evaluation of stress corrosion cracking resistance, the pH of the test solution 2 is preferably neutral to alkaline, but in the present invention, the pH of the test solution 2 is set to 10 (± 0.2) in consideration of better reproducibility. adjust.

  The temperature of the test solution 2 is not particularly limited, but room temperature is recommended. In the present invention, the temperature is set to 30 ° C. in consideration of better reproducibility. If the temperature of the aqueous solution is too low, the crack growth of stress corrosion cracking is slow, which is disadvantageous for promotion. On the other hand, if it is too high, there is a problem such as evaporation of the solution, which requires cooling. .

Data (current and potential) shall be collected at 1 point per 1 mV or 1 point per ^second , and the average gradient shall be calculated by reading the potential at which the current density has reached 1 to 10 mA / cm ² . In consideration of variations in data (current, potential), it is preferable to measure the gradient (current, potential) as described above five times or more and take an average value.

(Aluminum alloy material)
The evaluation method of the stress corrosion cracking resistance of the aluminum alloy material of the present invention covers various aluminum alloys such as 1000 series, 2000 series, 3000 series, 4000 series, 5000 series, 6000 series, 7000 series, and the like, and These aluminum alloy forged materials, rolled materials, extruded materials such as extruded materials can be targeted.

(Al-Mg-Si Al alloy material)
However, among these aluminum alloy materials, there are many conditions for stress corrosion cracking, the usage environment (application) is severe, and the stress corrosion cracking resistance is more demanded, as automotive structural members (panels, profiles, forgings). It is preferably applied to an Al—Mg—Si (60000) Al alloy material. In addition, among them, there are many stress corrosion cracking conditions in the usage environment such as high stress applied and exposure to salt water environment, and more severe stress corrosion cracking resistance is required. It is preferably applied to an Al—Mg—Si based Al alloy forging as an underbody member (security part, suspension arm, etc.).

In the Al-Mg-Si Al alloy material of the present invention, in order to improve the stress corrosion cracking resistance, according to the above measurement and evaluation method, 80% of the 0.2% proof stress value of this aluminum alloy material was loaded. The anodic polarization curve in a 5.8 mass% NaCl aqueous solution adjusted to pH 10 at a temperature of 30 ° C. was measured by the three-electrode method, and the current density of the anodic polarization curve m from 1 A / cm ² was measured. The average current / potential gradient in the range up to 10A / cm ² is 350 Ω ^-1 · m ^-2 or less.

  In addition, in the Al-Mg-Si based Al alloy forging of the present invention, in order to improve the stress corrosion cracking resistance after satisfying the basic required characteristics such as the required strength and toughness as the automobile undercarriage member, It is preferable to set it as the component composition.

  That is, as the Al-Mg-Si-based Al alloy forging material of the present invention, in mass%, Mg: 0.30-5.0%, Si: 0.20-2.0%, Cu: 0.01-2.0%, Mn: 0.01-1.0%, Fe: 0.01 to 1.0%, Cr: 0.01 to 2.0%, Zn: 0.005 to 10.0%, respectively, and the balance is Al and inevitable impurities. Further, this Al-Mg-Si-based aluminum alloy forging material is further used in terms of mass% to refine crystal grains in terms of mass%, Ti: 0.001 to 0.5%, B: 0.0001 to 0.05%, Nb: 0.01 to 1.0. %, Zr: 0.01 to 1.0%, V: 0.01 to 1.0%, or one or more may be contained.

(Each element amount)
Next, critical contents and preferable ranges of the contents of the respective elements of the Al alloy forged material of the present invention will be described below.

Mg: 0.30-5.0%
Mg precipitates as Mg ₂ Si (β 'phase) together with Si by artificial aging, and is an essential element for imparting high strength (yield strength) when the final product is used. If the Mg content is less than 0.30%, the age hardening amount decreases. On the other hand, if the content exceeds 5.0%, the strength (yield strength) becomes too high and the forgeability is impaired. In addition, a large amount of Mg ₂ Si is likely to precipitate during the quenching after the solution treatment, thereby reducing the corrosion resistance and toughness. Therefore, the Mg content is in the range of 0.30 to 5.0%, preferably in the range of 0.45 to 4.0%, and more preferably in the range of 0.6 to 3.0%.

Si: 0.20-2.0%
Si, together with Mg, is an essential element for precipitating as Mg ₂ Si (β ′ phase) by artificial aging treatment and imparting high strength (yield strength) when the final product is used. If the Si content is less than 0.20%, sufficient strength cannot be obtained by artificial aging. On the other hand, if the content exceeds 2.0%, coarse single Si particles crystallize and precipitate during casting and during quenching after solution treatment, and as described above, corrosion resistance and toughness are reduced. Furthermore, workability is also hindered, for example, elongation becomes low. Therefore, the Si content is in the range of 0.20 to 2.0%, and within this range, it is preferable to reduce the excess Si as much as possible in relation to the Mg content.

Cr: 0.01-2.0%
Cr forms dispersed particles (dispersed phase) such as Al ₁₂ Mg ₂ Cr and Al—Cr during homogenization heat treatment and subsequent hot forging. Since these dispersed particles have an effect of hindering grain boundary movement after recrystallization, fine crystal grains and sub-crystal grains can be obtained. This refinement of crystal grains and subgraining have a great effect of improving fracture toughness and fatigue characteristics. If the Cr content is too low, these effects cannot be expected.On the other hand, excessive Cr content can be dissolved and coarse Al-Fe-Si- (Mn, Cr, Zr) based intermetallic compounds and crystals during casting. Precipitates are easily generated, become the starting point of fracture, and cause a decrease in toughness and fatigue characteristics. Therefore, high toughness and high fatigue characteristics cannot be obtained. For this reason, the Cr content is in the range of 0.01 to 2.0%.

Cu: 0.01-2.0%,
In addition to contributing to strength improvement by solid solution strengthening, Cu also has the effect of significantly accelerating age hardening of the final product during aging treatment, and is essential for high strength. However, Cu significantly increases the susceptibility to stress corrosion cracking and intergranular corrosion of the structure of the Al alloy forging, and reduces the corrosion resistance and durability of the Al alloy forging. Therefore, in the present invention, from these viewpoints, the Cu content is in the range of 0.01 to 2.0%, preferably in the range of 0.05 to 1.5%, and more preferably in the range of 0.10 to 1.0%.

Mn: 0.01-1.0%
Mn generates Al-Mn-based dispersed particles such as Al ₂₀ Cu ₂ Mn ₃ during the homogenization heat treatment and the subsequent hot forging. There is an effect of obtaining crystal grains. In addition, an increase in strength and Young's modulus due to solid solution can be expected. However, Mn, on the other hand, produces Al-Fe-Si- (Mn, Cr, Zr) based crystal precipitates, so that if Mn content is high, corrosion resistance and toughness are lowered. Therefore, in the present invention, from these viewpoints, the Mn content is in the range of 0.01 to 1.0%, preferably in the range of 0.02 to 0.8%, and more preferably in the range of 0.03 to 0.6%.

Fe: 0.01-1.0%
Fe contained in the Al alloy has an effect of refining crystal grains and is effective in increasing the strength. However, Fe, on the other hand, is Al ₇ Cu ₂ Fe, Al ₁₂ (Fe, Mn) ₃ Cu ₂ , (Fe, Mn) Al ₆ , or coarse Al—Fe—Si— (Mn, Cr, Zr) system. To produce a crystalline precipitate. These crystal precipitates deteriorate fracture toughness and fatigue characteristics. Therefore, in the present invention, from these viewpoints, the Fe content is in the range of 0.01 to 1.0%, preferably in the range of 0.02 to 0.3%. In particular, by making the Fe content 0.30% or less, more strictly 0.25% or less, the total area ratio of Al-Fe-Si- (Mn, Cr, Zr) -based crystal precipitates is reduced per unit area. It can be 1.5% or less, preferably 1.0% or less, and can obtain higher strength and high toughness required for a structural material of a transport aircraft.

Zn: 0.005 to 10.0%
Zn deposits MgZn ₂ finely and densely during artificial aging to achieve high strength. In addition, Zn in solid solution lowers the potential in the grain, and the corrosion form is not from the grain boundary, but as an overall corrosion, and the effect of reducing the grain boundary corrosion and stress corrosion cracking can be expected. If the Zn content is less than 0.005%, sufficient strength cannot be obtained by artificial aging, and there is no effect of improving the corrosion resistance. On the other hand, if the content exceeds 10.0%, the corrosion resistance is remarkably lowered. Therefore, the Zn content is in the range of 0.005 to 10.0%.

  As other elements, one or two or more selected from Ti, B 2, Nb, Zr, and V may be selectively contained in order to refine crystal grains.

Ti: 0.001 to 0.5%.
Ti is an element added to refine crystal grains of an ingot and improve workability during extrusion, rolling, and forging. However, if the Ti content is less than 0.001%, the effect of improving the workability cannot be obtained. On the other hand, if the Ti content exceeds 0.5%, coarse crystal precipitates are formed and the workability is lowered. Therefore, the Ti content is preferably in the range of 0.001 to 0.5%.

B: 0.0001-0.05%
B, like Ti, is an element added to refine the ingot crystal grains and improve the workability during extrusion, rolling and forging. However, when the content of B is less than 0.0001%, this effect cannot be obtained. On the other hand, when the content exceeds 0.05%, coarse crystal precipitates are formed and the workability is lowered. Therefore, the B content is preferably in the range of 0.0001 to 0.05%.

Nb: 0.01-1.0%
Nb is also an element added to refine crystal grains of the ingot and improve workability during extrusion, rolling, and forging. However, when the content is less than 0.01%, this effect cannot be obtained. On the other hand, when the content exceeds 1.0%, coarse crystal precipitates are formed, and the workability is lowered. Therefore, the Nb content is preferably in the range of 0.01 to 1.0%.

V: 0.01-1.0%
V produces dispersed particles (dispersed phase) during the homogenization heat treatment and the subsequent hot forging, as in the Mn, Cr, Zr system and the like. Since these dispersed particles have an effect of hindering grain boundary movement after recrystallization, fine crystal grains and sub-crystal grains can be obtained. When the content is less than 0.01%, this effect cannot be obtained. On the other hand, when the content exceeds 1.0%, Al-Fe-Si-V-based coarse intermetallic compounds and crystal precipitates are also produced during melting and casting. Forming a starting point of fracture and reducing toughness. Therefore, the V content is preferably in the range of 0.01 to 1.0%.

Zr: 0.01-1.0%,
Zr also generates fine dispersed particles (dispersed phase) such as Al-Zr system during homogenization heat treatment and subsequent hot forging. Since these dispersed particles have the effect of hindering the grain boundary movement after recrystallization, fine crystal grains and sub-crystal grains can be obtained. When the content is less than 0.01%, this effect cannot be obtained.On the other hand, when the content exceeds 1.0%, coarse intermetallic compounds and crystal precipitates are formed during melting and casting, which becomes the starting point of fracture. , Reduce toughness. Therefore, the Zr content is preferably in the range of 0.01 to 1.0%.

(Al alloy forging production method)
Next, although the Al alloy material in the present invention can be manufactured according to a conventional method, a preferable method for manufacturing a forged material will be described below. Although the production of the Al alloy forged material itself can be performed by a conventional method, preferable conditions for obtaining required characteristics such as strength, toughness, and corrosion resistance necessary for the undercarriage parts will be described below.

  First, in the case of casting an Al alloy melt whose melting is adjusted within the range of the Al alloy component, for example, a normal melt casting such as a continuous casting rolling method, a semi-continuous casting method (DC casting method), a hot top casting method, etc. The method is appropriately selected and cast.

  Here, in order to refine the crystal grains of the Al alloy ingot and control the Al-Fe-Si- (Mn, Cr, Zr) -based crystal precipitates present on the grain boundaries, an Al alloy molten metal is used. The ingot is preferably cast at a cooling rate of 15 ° C./sec or more. By setting the cooling rate of the ingot to 15 ° C./sec or more, the crystal grains of the Al alloy ingot are refined, and mechanical properties such as strength and toughness are improved. On the other hand, if the cooling rate is slower than 15 ° C / sec, the Al-Fe-Si- (Mn, Cr, Zr) -based crystal precipitates present on the grain boundaries are coarsened, and the crystal grains are coarsened. As the dendrite secondary arm spacing (DAS) increases, strength, toughness and corrosion resistance decrease.

Next, the homogenization heat treatment temperature of the Al alloy ingot (cast material) is preferably in the temperature range of 500 to 560 ° C. The normal homogenization heat treatment temperature of this kind of Al alloy cast material is usually about 480 to 580 ° C. However, in the present invention, in order to improve the corrosion resistance and toughness, the Al-Fe-Si- (Mn, Cr, Zr) Mg ₂ Si or Al-Fe-Si- (Mn, Cr, Zr) present on the grain boundaries of the forged structure after sufficiently crystallization and precipitation of the system crystal precipitates ) It is preferable to refine the system crystal precipitate.

For this purpose, the above-mentioned homogenization heat treatment at a high temperature of 500 to 560 ° C. is necessary. When the homogenization heat treatment temperature is less than 500 ° C., Al—Fe—Si— (Mn, Cr, Zr) crystal precipitates However, Mg ₂ Si and Al—Fe—Si— (Mn, Cr, Zr) based crystal precipitates present on the grain boundaries of the structure of the forged material after the tempering treatment are coarsened. On the other hand, if the homogenization heat treatment temperature exceeds 560 ° C., on the other hand, burning (melting damage) or the like occurs in the Al alloy ingot (cast material), and cracking is likely to occur during hot working. In addition, mechanical properties such as toughness and fatigue properties of the final forged material may be significantly reduced.

  After the homogenization heat treatment, hot forging is performed by die forging to form an Al alloy forging material having a final product shape (near net shape) such as the undercarriage part. And after forging, T6 (artificial age hardening treatment to obtain maximum strength after solution treatment) to obtain the required strength, toughness and corrosion resistance, T7 (artificial age hardening treatment to obtain maximum strength after solution treatment) Excessive aging treatment exceeding conditions), T8 (artificial aging hardening treatment for obtaining maximum strength by performing cold working after solution treatment, and further appropriate treatment) and the like are appropriately performed. Moreover, an air furnace, an induction heating furnace, a nitrite furnace, etc. are suitably used for the homogenization heat treatment and the solution treatment. Furthermore, an air furnace, an induction heating furnace, an oil bath, or the like is appropriately used for the artificial age hardening treatment.

  The temperature of the artificial age hardening treatment for obtaining the maximum strength in the tempering treatment is in the range of 175 to 200 ° C, preferably in the range of 180 to 195 ° C. When this temperature is too low, the processing time for obtaining the required strength becomes long, and the stress corrosion cracking property is lowered. On the other hand, if this temperature is too high, the precipitates become coarse and the strength decreases.

  Examples of the present invention will be described below. A 6000 series Al alloy ingot having the composition shown in Table 1 was cast at each cooling rate shown in Table 2 by the hot top casting method. This ingot was subjected to a homogenizing heat treatment at each temperature shown in Table 2 for 8 hours, and after reheating, it was hot forged by mechanical forging to produce a plate-like Al alloy forged material having a thickness of 20 mm. This forged material is heated in an air furnace at a heating rate of 300 ° C / hr, and after common solution treatment at 540 ° C for 1 hour, water cooling (water quenching) is performed, and then room temperature (20-30 ° C) ) For 1 hour, and then an aging treatment (T6 treatment) for 8 hours at each temperature shown in Table 2.

(Mechanical characteristics measurement)
The mechanical properties, tensile strength (σ _B , MPa), proof stress (σ _0.2 , MPa), elongation (δ,%) of these manufactured Al alloy forgings are defined as the direction perpendicular to the plate thickness direction. JIS No. 5 tensile test specimens were collected and measured according to JIS Z 2201. The crosshead speed was 5 mm / min, and the test was performed at a constant speed until the test piece broke. Each sample was tested three times and the average value was adopted.

(Average slope measurement of current / potential)
Specimens were taken from each of the aluminum alloy forgings, processed into the C-ring specimens shown in FIG. 3, and bolted to obtain the ratios (%) shown in Table 2 for the 0.2% proof stress value of the aluminum alloy forgings. A test piece loaded with stress was prepared as a C-ring type anode electrode test piece 3. This test piece was covered with silicon sealant except the width of 10mm across the apex (maximum stress point) to prevent the bolts and nuts from being exposed to electrolysis.

These test pieces were commonly pretreated under the conditions described above, and each C-ring type anode electrode test piece 3 loaded with each stress (ratio:%) shown in Table 2 using the apparatus shown in FIG. Was immersed in NaCl aqueous solution (test aqueous solution). Table 2 shows the temperature and pH conditions of the NaCl aqueous solution. Then, the anodic polarization curve of each C-ring type anode electrode test piece 3 was measured by the three-electrode method, and the average gradient of current / potential (Ω ⁻¹⁾ in the current density range from 1 A / cm ² to 10 A / cm ^{2. -M -2} ) was measured. The potential feed rate at this time was 50 mV / min.

  As test solution conditions, Examples 1 to 13 in Table 2 were 5.8 mass% NaCl aqueous solution adjusted to pH 10 at 30 ° C., which is the condition of the present invention. On the other hand, in Examples 16 to 19 in Table 2, the correlation with stress corrosion cracking was compared as a 5.8 mass% NaCl aqueous solution in which the temperature and pH were intentionally removed from the conditions of the present invention. In Examples 14 and 15 of Table 2, relatively low stresses of 60% and 40% of the 0.2% proof stress value of the aluminum alloy forging were applied, and the correlation with stress corrosion cracking was compared.

(Stress corrosion cracking evaluation)
Furthermore, in order to see the relationship between this potential difference and stress corrosion cracking, test specimens collected from each of the Al alloy forgings (test specimens collected from the site adjacent to the potential difference measurement specimen collection position) were subjected to stress corrosion. The stress corrosion cracking property was evaluated by an alternate immersion test method described in JISH8711, which is widely used as a cracking property evaluation test.

  In this alternate immersion test method, the sample is immersed for 10 minutes in a 3.5% by weight NaCl aqueous solution (pH 6.8, 25 ° C) and then taken out for 50 minutes in a constant temperature and humidity atmosphere at 25 ° C and 50% RH. The process of drying was repeated. Then, the number of days (hours) until stress corrosion cracking occurred was measured by observing the surface of the specimen once / day. X where stress corrosion cracking occurred within 30 days, △ where stress corrosion cracking occurred between 31 and 90 days, and ○ and 181 where stress corrosion cracking occurred between 91 and 180 days Those in which stress corrosion cracking did not occur for more than a day were evaluated as ◎. These results are shown in Table 2.

FIG. 4 shows the relationship between the average current / potential gradient (Ω ⁻¹ · m ⁻² ) in the anodic polarization curve and the stress corrosion cracking evaluation result by the alternate immersion test method. In Fig. 4, the vertical axis is the number of days until the occurrence of stress corrosion cracking, the horizontal axis is the average gradient of current / potential (Ω ^-1・ m ^-2 ), and the vertical line is the average current / potential gradient of 350 It is the place of Ω ^-1 · m ^-2 . In FIG. 4, the arrows above Examples 1 and 2 in Table 2 indicate that the number of days until stress corrosion cracking in Examples 1 and 2 cannot be plotted on the vertical axis.

  As is clear from FIG. 4, the average current / potential gradient measured under the test conditions of the present invention corresponds well (correlation) with the days until stress corrosion cracking in Examples 1 to 13 in Table 2.

  In contrast, Examples 14 and 15 in Table 2 have the same alloy composition and manufacturing conditions as the forged material, but differ only in the measurement conditions for the average gradient of current / potential in the above anodic polarization curve. . That is, only the stress load on the test piece (electrode 3) was changed, and a relatively low stress of 60% and 40% of the 0.2% proof stress value of the aluminum alloy forging was applied.

Therefore, as in Example 10 of Table 2, the average slope of current / potential in the above anodic polarization curves of Examples 14 and 15 in Table 2 is 350, although the actual forging has low stress corrosion cracking property. Ω ⁻¹ · m ⁻² or less, which satisfies the scope of the present invention. Therefore, the actual stress corrosion cracking resistance does not correlate with the tendency of the average gradient of current / potential in the anodic polarization curve, and cannot be used as an evaluation method.

  On the other hand, Examples 16 to 19 in Table 2 are 5.8 mass% NaCl aqueous solutions in which the temperature and pH are deliberately excluded from the conditions of the present invention. As a result, although the correlation with stress corrosion cracking is the same as in Example 3 where the temperature and pH are within the range of the present invention, the average current / potential gradient in the above anodic polarization curve is similar to that of Example 3 and Example 3. Slightly different.

  Therefore, these results show the effectiveness of stress corrosion cracking resistance evaluation based on the average current / potential gradient in the anodic polarization curve. It also shows the significance of each measurement condition for the average current / potential gradient in the anodic polarization curve for measurement reproducibility.

Further, Examples 1 to 8 in Table 2 are produced by the above-described preferred production method as described in Table 2 within the component composition range of the forging material of the present invention (alloy numbers A to H in Table 1). Yes. As a result, Examples 1 to 8 in Table 2 were excellent in stress corrosion cracking resistance, and the anodic polarization curve in a 5.8 mass% NaCl aqueous solution adjusted to pH 10 at 30 ° C. was measured by the three-electrode method. In the current density range of 1 A / cm ² to 10 A / cm ² , the average current / potential gradient is 350 Ω ^-1 · m ^-2 or less.

On the other hand, within the component composition range of the forged material of the present invention, as shown in Table 2, the casting cooling rate is too low Example 9, the artificial aging temperature is too low Example 10, the homogenization treatment temperature is too high Example 11, artificial aging In Example 12 where the temperature is too low, the average gradient of the current / potential exceeds 350 Ω ⁻¹ · m ⁻² , and the resistance to stress corrosion cracking is poor. In Example 13, the artificial aging temperature is too high, the average current / potential gradient is less than 350 Ω ^-1 m- ² , and the stress corrosion cracking resistance is excellent, but compared to the preferred artificial aging temperature example 8. The strength is considerably lowered, and the strength is not improved by the addition of alloy elements.

Therefore, from these results, the critical significance of the current / potential average slope value 350 Ω ⁻¹ · m ^{−2 in} the anodic polarization curve in the 6000 series forged material of the present invention to the stress corrosion cracking resistance can be understood.

  According to the present invention, there is provided a more improved evaluation method for the correspondence with the stress corrosion cracking resistance behavior of an actual aluminum alloy material and an aluminum alloy material excellent in stress corrosion cracking resistance evaluated by this evaluation method. it can. As a result, the application of the 6000 series aluminum alloy material can be expanded for uses such as automobile parts and members.

It is a conceptual diagram which shows the anode polarization curve measuring apparatus which concerns on this invention. It is explanatory drawing which shows the anodic polarization curve measured with the apparatus of FIG. It is explanatory drawing which shows the C ring test piece (electrode) shape used for an anodic polarization curve measurement. It is explanatory drawing which shows the correlation with the time until stress corrosion crack generation in an Example, and the average gradient value of the electric current / potential in an anodic polarization curve.

Explanation of symbols

1: container, 2: NaCl aqueous solution, 3: cathode electrode (test piece), 4: counter electrode,
5: reference electrode, 6: polarization measuring device

Claims (4)

A method for evaluating the resistance to stress corrosion cracking of an aluminum alloy material, in which 5.8 mass% NaCl adjusted to a temperature of 30 ° C. and pH 10 in a state where a predetermined stress is applied to the aluminum alloy material test piece to be evaluated. The anodic polarization curve in an aqueous solution was measured by the three-electrode method, and the current density of the measured anodic polarization curve in the range from 1 A / cm ² to 10 A / cm ² was determined by the current / potential average gradient. An evaluation method for stress corrosion cracking resistance of an aluminum alloy material, characterized by evaluating stress corrosion cracking resistance of the material. An Al-Mg-Si aluminum alloy material whose stress corrosion cracking resistance has been evaluated by the evaluation method according to claim 1, wherein a stress of 80% of the 0.2% proof stress value of the aluminum alloy material In the state of anodic polarization in a 5.8 mass% NaCl aqueous solution adjusted to a temperature of 30 ° C. and a pH of 10, measured by the three-electrode method, the current density was 1 A / cm ² to 10 A / cm ² An aluminum alloy material excellent in stress corrosion cracking resistance, characterized by an average current / potential gradient in the range up to 350 Ω ^-1 · m ^-2 or less.   The Al-Mg-Si-based aluminum alloy material is, by mass%, Mg: 0.30 to 5.0%, Si: 0.20 to 2.0%, Cu: 0.01 to 2.0%, Mn: 0.01 to 1.0%, Fe: 0.01 to 1.0 The aluminum alloy material having excellent stress corrosion cracking resistance according to claim 2, which is a forging material comprising:%, Cr: 0.01 to 2.0%, Zn: 0.005 to 10.0%, and the balance Al and inevitable impurities.   The Al-Mg-Si based aluminum alloy forged material is further, in mass%, Ti: 0.001 to 0.5%, B: 0.0001 to 0.05%, Nb: 0.01 to 1.0%, Zr: 0.01 to 1.0%, V: The aluminum alloy material excellent in stress corrosion cracking resistance according to claim 3, containing one or more selected from 0.01 to 1.0%.

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